As a method for forming a visible image on a recording medium according to electric image signals, a method called "xerography" has been generally known. In this method, an electrostatic pattern (or electrostatic latent image) is formed on an image carrier having a photosensitive layer with electro-optical properties by optical writing means. After developing the electrostatic pattern to form a toner image by causing toner (developer particles) to adhere to the electrostatic pattern, the toner image is transferred onto the recording medium. Thus, the image signals are developed as a visible image on the recording medium.
In this method, first, an image signal is converted into an optical signal using a light emitting device (or light emitting element) such as a laser and LED (light emitting diode). Then, the optical signal is applied to the photosensitive layer which has been uniformly charged beforehand, thereby forming an electrostatic pattern according to the light intensity. Next, a toner image is formed on the photosensitive layer by bringing charged toner into contact with the electrostatic pattern or causing the toner to fly to the electrostatic pattern (this process is called the development step). Subsequently, the toner on the image carrier is caused to adhere to the recording medium by electric adhesion and/or pressure (this process is called the transfer step). Thereafter, the toner is fixed to the recording medium by applying pressure and/or heat to the recording medium.
Another known method uses a charged particle generator, a charged particle flow control grid, and a dielectric drum as the image carrier. In this method, first, a charged image corresponding to image signals is formed on the dielectric drum by controlling the flow of charged particles according to the image signals. Next, a toner image is formed on the image carrier by developing the charged image using toner. Subsequently, the toner on the image carrier is caused to adhere to a recording medium by electric adhesion and/or pressure. Thereafter, the toner is fixed onto the recording medium by applying pressure and/or heat to the recording medium.
In the above-mentioned conventional methods, the image signals are formed as an electrostatic latent image temporarily on the image carrier, and then the toner image is formed on the image carrier by developing the electrostatic latent image using the toner. It is therefore necessary to use the image carrier having a special structure, and writing means for writing the electrostatic latent image. Additionally, when a repeat use of the image carrier is required, it is necessary to provide means for erasing a previously written electrostatic latent image as well as the writing means. Moreover, since the toner image formed temporarily on the image carrier is caused to adhere to the recording medium, the processing steps tends to be complicated, and problems arise in terms of the stability and miniaturization of the apparatus.
In order to solve these problems, for example, U.S. Pat. No. 5,036,341 (corresponding to Japanese publication of international patent application No. 50321/1989 (Tokuhyohei 1-50321)) proposes a direct image forming method. In this method, a toner image is formed on the recording medium by causing charged toner (developer particles) to fly to the recording medium directly and selectively using a charged particle flow control grid (control electrode) that is controlled according to image signals. Thereafter, the toner image is fixed to the recording medium by applying pressure and/or heat to the recording medium. In this method, since the above-mentioned image carrier is not used, the processing steps are simplified, thereby achieving a compact apparatus with improved stability.
However, in the device of the above-mentioned patent, when a signal voltage corresponding to an image to be formed is intermittently applied to the control electrode in forming the toner image on the recording medium, the electrostatic force acting in the vicinity of the control electrode varies. As a result, the control electrode vibrates. When a standing wave is generated by resonance of the vibration of the control electrode and another vibration produced in the image forming apparatus, the standing wave produces noise, preventing a quiet apparatus. In addition, the resonance displaces the control electrode, and the position to which the toner flies deviates from an area where the image is to be formed. Thus, there is a possibility that the image quality is lowered.
In order to prevent noise from being generated during image formation, Japanese publication of unexamined patent application No. 15980/1996 (Tokukaihei 8-15980) discloses a method of suppressing vibrations by offsetting a force produced by a voltage applied to the electrode by the application of a counter electric field. However, in the direct image forming method, since the flying of toner is controlled by an electric field in the vicinity of the control electrode, it is theoretically impossible to apply excessive strength of counter electric field to the vicinity of the control electrode. Therefore, this method cannot be used as means for preventing noise in the direct image forming method.
Furthermore, Japanese publication of unexamined patent application No. 71928/1994 (Tokukaihei 6-71928) discloses a recording device in which the displacement due to vibrations is decreased by placing an opening of an aperture electrode at a position where the value of vibration becomes minimum. This arrangement can improve the accuracy of the position of the control electrode. However, in this device, although the position of the opening is fixed, there is a possibility that the position showing the minimum vibration varies with a change in environment such as ambient temperature, and the type of the device. Hence, this recording device is instable. Additionally, in this patent document, any method for damping possible vibrations of an arbitrary frequency is not considered. Namely, this patent document does not propose a fundamental method for eliminating vibrations.